1. Field of the Invention
The present invention relates to active pixel sensors and in particular an active pixel sensor which approximately doubles the available signal for a given quanta of light.
2. Description of Related Art
A conventional Active Pixel Sensor (APS) cell consists of an np diode in the p-substrate, an NFET transfer device, a source-follower amplifying transistor, a pre-charge transistor, and a bit switch transistor. Electron-hole pairs are produced in the diode by impinging photons. The electrons are collected in the pre-charged n region of the diode, and eventually transferred to the source-follower gate for amplification. A p-type pinning layer is often included at the surface of the diode.
In APS cells, pn diodes collect electrons generated by incoming light. The electron charge is amplified and the signal-to-noise ratio is proportional to the charge level. Holes generated by the light are not collected and recombine in either of the p-type terminals.
FIG. 1a illustrates a schematic cross-section of the photodiode and the transfer gate. FIG. 1b illustrates the schematic of the circuit embodied in each Active Pixel Sensor device 10. The number of electrons collected determines the level of gate drive on the source-follower transistor 12. The source potential is transferred down the column through the bit switch 14, where it can be processed appropriately for video imaging. In the conventional cell 10, holes equal in number to the electrons are created and are shunted to the substrate 16 creating a substrate current which exits the cell through the ground line.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an active pixel cell device for imaging whereby the size of the pixel cell is reduced.
It is another object of the present invention to provide an active pixel sensor which produces an output signal and a complementary output signal.
It is another object of the present invention to provide an active pixel sensor device with improved common-mode noise rejection.
It is another object of the present invention to provide an active pixel sensor device with improved signal-to-noise ratio.
It is another object of the present invention to provide an active pixel sensor device with reduced fixed pattern noise.
It is another object of the present invention to provide an active pixel sensor device with a reduced size collection area.
A further object of the invention is to provide an active pixel sensor device which approximately doubles the output current relative to conventional active pixel sensor devices.
It is yet another object of the present invention to provide an active pixel cell device which virtually eliminates substrate current.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.